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NEW YORK ( TheStreet) -- Citigroup(C) investors and analysts are narrowing their focus on the bank's massive deferred tax asset (DTA), which is growing despite three consecutive years of profits.

The DTA reflects losses and other deductions that get converted to tax credits. However, Citigroup must generate sufficient profits to make use of the credits before they expire, an issue causing anxiety among some followers of the stock.

Citigroup CFO John Gerspach was asked about the issue by two attendees during a presentation at a conference hosted by Credit Suisse on Tuesday after he noted that $40 billion of the bank's $151 billion in tangible common equity must be used to support the DTA and so cannot be counted for regulatory purposes under the rules known as Basel III.

Neither can the $40 billion be used to generate a return. Citigroup earned a 10.7% return on its remaining $111 billion in tangible common equity in 2012, so if it could have earned that return on the other $40 billion, it could have improved earnings by $4.28 billion.

Gerspach on Tuesday attributed the growth in the DTA to "one-offs" (a euphemism for irregular items that are unlikely to recur), though he gave little indication about when the DTA would begin to come down.

The DTA has grown to $55 billion, up from $38 billion in 2009 and $50 billion in 2010. The fact that it is so large and continues to grow is "very, very troubling," according to Robert Willens, an independent tax consultant who has been critical of Citigroup's tax-related accounting strategies in the wake of the 2008 crisis.

Gerspach attributed the rise to the losses in its Citi Holdings' "bad bank" unit, including an impairment on its stake in wealth-management venture Morgan Stanley Smith Barney, all of which he said added $4.5 billion to the DTA. Tacking on another $1.3 billion was a fourth-quarter "repositioning charge" and an accounting oddity known as a debt valuation adjustment in which a bank's improving creditworthiness actually has a negative impact on earnings.

Gerspach added that "there are elements of the DTA that come about because of actual tax returns that are filed, you can see that mostly in the foreign tax credits, and we'll give you a full rundown of that when we publish the 10K in a couple of weeks, but then there is also a whole series of timing-related issues, and that mostly comes about as a result of reserve movements," according to a transcript of Tuesday's call.

After reviewing the sections of the transcript dealing with the DTA, tax expert Willens said he could make little sense of Gerspach's explanation.

"The only thing I could even remotely comprehend was I think he was trying to explain why the DTA was going up," Willens said.